In known vehicle speed control systems, typically referred to as cruise control systems, a set-speed for the vehicle may be initially set by manually bringing the vehicle up to the desired speed, and then manipulating a user-selectable user interface device, such as, for example, by manipulating (e.g., pressing) a pushbutton to set that speed as the set-speed. When the user wants to change the set-speed thereafter, the same or different user input device(s) may be manipulated to increase or decrease the set-speed. In response to a requested or commanded change in set speed, the speed control system causes the vehicle to accelerate or decelerate, as appropriate, to reach or match the new set-speed by sending commands to one or more vehicle subsystems, such as, for example, the powertrain and/or brake subsystems of the vehicle.
One drawback of such known speed control systems, however, is that when the vehicle speed falls below the target set-speed or a change in set-speed to a target set-speed is commanded and the vehicle is accelerated or decelerated to reach the target set-speed, forces external to the vehicle that may affect vehicle acceleration may not be sufficiently compensated for. As a result, the vehicle may be accelerated or decelerated in a manner that may cause the overshooting or undershooting of the target set-speed, as well as causing the vehicle acceleration or deceleration to be inconsistent with one or more of the user's expectation (e.g., it may accelerate/decelerated faster or slower than the user anticipates or expects), a prescribed acceleration profile, and predetermined acceleration corridor. These forces may include, for example, gravitational forces acting on the vehicle as the vehicle ascends or descends a gradient, rolling resistance resulting from a high drag surface the vehicle is traversing, drag associated with driving through a body of water (i.e., during a wading event), and the like. If the magnitude of one or mere forces acting on the vehicle is sufficiently high, the acceleration or deceleration of the vehicle, as the case may be, may be adversely affected in mat the force may undesirable increase or decrease the acceleration or deceleration of the vehicle beyond that which would be expected.
Accordingly, there is a need for a speed control system and method for use with the same that minimizes and/or eliminates one or more of the above-identified deficiencies.